1. Field of the Invention
The present invention is in the field of energy storage. More specifically, the present invention is directed various embodiments of an energy storage device and methods of using the device to store energy mechanically using gravitational potential by utilizing the height difference between the bottom and the surface of a deep body of water.
2. Description of Related Art
As more and more sources of intermittent generation from renewable sources like the wind and sun are integrated into the electric grid, the critical task of matching generation output to electrical demand for which grid operators are responsible becomes more and more difficult. Electricity must be used exactly when it is delivered, so grid operators must continuously adjust how much energy is being produced at each moment to exactly match the amount of energy its end-users require. Grid operators must make use of generators that can be throttled, such as natural gas combustion turbines, to adjust total generation output up and down. Running combustion turbines at varying speeds, however, is inefficient and causes excess wear and tear on the machinery.
As a larger and larger fraction of the world's energy is derived from intermittent sources, more and more combustion turbines running under these non-optimal conditions will be required, presenting an ever-larger and more costly problem to utilities. One solution to this problem is to store energy from intermittent sources from when it is generated until it is needed by the end users. Storage has the potential to turn intermittent renewable energy sources into a source of energy that can be dispatched on demand. To turn renewables into a dispatchable resource, however, requires a huge amount of storage.
Existing energy storage technologies fall short of being able to perform this function economically. The ubiquitous lead-acid battery has a low sticker price but is inefficient and limited in its usable depth of discharge. It also has short cycle life, which requires it to be replaced often. Advanced flow and lithium-ion batteries improve upon the cycle-life and usable depth of discharge but are very expensive and still face the issue of degrading performance over time. Flywheels are effective at storing energy for short-duration, high-power applications and are used on the grid today to perform this task, but their reliance on power-sapping electromagnetic bearings, actively pumped down vacuum enclosures, and expensive composite materials make them unsuitable for storing large amounts of energy for many hours. Ultracapacitors also rely on advanced materials and are only suitable for use in short-duration, high-power applications. Existing compressed air energy storage (CAES) systems store large amounts of energy but require combustion of substantial amounts of natural gas to recover this stored energy. Further, they suffer from low round-trip efficiencies (50-70%). They also rely on the existence of special underground salt caverns to provide a container for the compressed air, thereby limiting available sites.
One technology that may provide economical energy storage on a massive scale is pumped hydro. Pumped hydro stores energy using gravitational potential. It takes excess energy when it is generated and uses it to pump water from a lower reservoir hundreds of feet up to a higher reservoir. When the energy is needed again, the water is allowed to flow back down to the lower reservoir and in the process spins a turbine that in turn spins a generator, which converts the energy back into electrical form. Although pumped hydro is expensive to install, it is a relatively low-cost way to store energy in the long run since its efficiency is 70-85%. Unfortunately, pumped hydro relies on the existence of a mountain with a suitable site for both an upper and lower reservoir to be economical. The majority of desirable sites for pumped hydro have already been developed, and developing new sites has serious environmental consequences. For these reasons, it is likely that there will be few, if any, new pumped hydro storage facilities installed in the future.
Based on the foregoing, it would be desirable to have an energy storage device that can store energy and release the stored energy reliably and economically. In addition, it would be desirable to provide an energy storage system that can store energy from various energy generation sources, such as offshore generation from wind turbines, and provide that stored energy to an electrical load, such as an electric grid.